Flexible conduit with pressure vault interlocked from below

ABSTRACT

A flexible tubular pipe comprising at least, from the inside outwards, an internal sealing sheath made of a polymer material, a cylindrical pressure vault having an external face and having an internal face placed over the internal sheath. The vault is a winding, in a helix with a short pitch and with a gap between turns, of an interlocked metal profile wire. At least one ply of tensile armour layers is wound with a long pitch. An external protective sealing sheath is made of a polymer. The fastener wire has the same height as the profile wire and is fastened below the neutral fiber of the profile wire. The ratio of the cross section of the profile wire to the cross section of the fastening wire is between 1 and 2 and the ratio of the moment of inertia I yy  of the profile wire to the moment of inertia I yy  of the fastening wire is between 1 and 2.

BACKGROUND OF THE INVENTION

The present invention relates to a flexible pipe for transporting, overlong distances, a fluid which is under pressure and possibly at a hightemperature, such as a gas, petroleum, water or other fluids. Theinvention relates most particularly to a pipe intended for offshore oilexploration. It is known that there are, on the one hand, bottom pipes,called flow lines, that is to say flexible pipes which are unwound froma barge in order to be laid generally on the bottom of the sea andconnected to the subsea installations, such pipes working mainly instatic mode, and, on the other hand, rising columns, called risers, thatis to say flexible pipes which are unwound from a surface installationsuch as a platform and are connected to the subsea installations andmost of which do not lie on or below the seabed, such pipes workingessentially in dynamic mode. The invention relates more particularly topipes working in dynamic mode.

The flexible pipes used offshore must be able to resist high internalpressures and/or external pressures and also withstand longitudinalbending or twisting without the risk of being ruptured.

They have various configurations depending on their precise use but ingeneral they satisfy the constructional criteria defined in particularin the standards API 17 B and API 17 J drawn up by the AmericanPetroleum Institute under the title “Recommended Practice for FlexiblePipe”.

A flexible pipe generally comprises, from the inside outwards:

-   -   an internal sealing sheath made of a plastic, generally a        polymer, resistant to a greater or lesser extent to the chemical        action of the fluid to be transported;    -   a pressure vault resistant mainly to the pressure developed by        the fluid in the sealing sheath and consisting of the winding of        one or more interlocked metal profile wires (which may or may        not be self-interlockable) wound in a helix with a short pitch        around the internal sheath;    -   at least one ply (and generally at least two crossed plies) of        tensile armour layers whose lay angle measured along the        longitudinal axis of the pipe is less than 55° C.; and    -   an external protective sealing sheath made of a polymer.

Such a structure is that of a pipe with a so-called smooth bore. In apipe with a so-called rough bore, a carcass consisting of an interlockedmetal strip is also provided inside the internal sealing sheath, servingto prevent the pipe collapsing under external pressure.

The pressure vault consists of a winding of non-touching turns so as togive the pipe a degree of flexibility. The expression “non-touchingturns” is understood to mean turns between which a certain space orinterstice, called hereafter “gap”, is left, which gap may be greaterthe larger the wound profile wire.

There are two types of pressure vault:

-   -   pressure vaults for static applications;    -   pressure vaults for dynamic applications.

For static applications, the pressure vaults only have to withstand theinternal and external pressures. They are not subjected to the fatiguecaused by the rubbing due to dynamic stressing. In this case, vaultwires with any type of interlocking may be used. The wire (cross sectionand inertia) is selected according to the internal and externalpressures.

For dynamic applications, such as those mainly intended by theinvention, the pressure vaults must withstand, in addition to theinternal and external pressures, large stresses due to dynamicstressing. These stresses are due to contacts between the wiresconstituting the vault. These contacts cause rubbing which, combinedwith the large bearing pressures, result in a reduction in the lifetimeof the profile wires by fatigue cracking.

This problem is encountered, for example in the pipe known from U.S.Pat. No. 4,549,581 which shows the interlocking by U-shaped fasteners ofthe U-shaped wires from below. These fasteners are too weak to withstandthe dynamic stressing. In fact, such a fastener withstands only littlestress by itself; it bends and bears on the outline of the profile wire.This results in bending moments and therefore alternating stresses, ofgreater or less magnitude according to the position and therefore of thelevel of dynamic stressing. In this case, the behaviour under dynamicconditions depends on the initial position of the profile wire withrespect to the fastener and on the level of dynamic stressing, andtherefore on the magnitude of the waves. The cross sections and thecharacteristics of the material must be adjusted according to the levelsof severity. This type of interlocking considerably limits theperformance of the pressure vault in dynamic applications.

This is why the Applicant has already proposed in document FR 2 727 738to raise the fastening region into the upper portion of the pressurevault. Since the interlocking is carried out from above, the fastener islittle exposed to the internal pressure (which is taken up by theprofile, in this case “teta”, wire) and to the amplitudes ofdisplacement due to the dynamic stressing.

According to document EP 0 431 142, which also relates to a pressurevault with a “teta” profile wire, the interlocking is done approximatelylevel with the neutral fibre.

In Patent U.S. Pat. No. 5,730,188, it has been proposed, in order toimprove the fatigue resistance in dynamic mode of “zêta” wires, to raisethe interlocking region into the upper portion of the vault and to makea chamfer on the upper flange of the wire. This profile wire makes itpossible to limit contacts in the interlocking region and thus increasesthe fatigue resistance of the pressure vault.

The Applicant has also proposed in document FR 2 783 142 to use alightened profile wire of large moments of inertia, in the form of an I,which, for dynamic applications, is also interlocked from above.

It may therefore be seen that, for dynamic applications, a consensus hasbeen established whereby the profile wire is interlocked exclusivelyfrom above.

For deepsea applications, it is sought to have moment of inertia at thepressure vault so as also to withstand the external pressure. In thiscase, the profile wire constituting the vault has a relatively largeheight which, in a dynamic application with interlocking from above,creates a large gap. The internal sheath may creep into this largevolume due to the effect of temperature and of the internal pressure,which results either in damage to the sheath or in disruption to theinterlocking of the pressure vault. This problem may be alleviated byproviding the pressure sheath with additional thickness, but thatincreases the cost of this sheath.

This is why the Applicant has also proposed, in document FR 2 782 141,anti-creep devices to mask these gaps. However, the use of these devicesincreases the cost of manufacturing these pressure vaults.

SUMMARY OF THE INVENTION

The object of the invention is to provide a pipe whose pressure vault iswell suited to withstanding deepsea dynamic stressing, without havingthe drawbacks of the known pipes.

The present invention achieves its objective by means of a flexibletubular pipe of the rough-bore or smooth-bore type, that is to saycomprising at least, from the inside outwards, an internal sealingsheath made of a polymer material, a cylindrical pressure vault havingan external face and an internal face placed over the internal sheath,the vault consisting of the winding, in a helix with a short pitch andwith a gap between turns, of a metal profile wire interlocked from belowby a fastener wire, at least one ply of tensile armour layers wound witha long pitch and an external protective sealing sheath made of apolymer, characterized in that the fastener wire has substantially thesame height as the profile wire and is interlocked, with no nominalradial contact, below the neutral fibre of the wire (that is to say thefibre where there is no elongation during spiralling, at the centre ofmass of the cross section of the wire). The expression “with no nominalradial contact” is understood to mean that the profile wire and thefastener wire are shaped with a nominal radial clearance at theirinterlocking regions. In practice, the interlocking region, defined bythe mean plane of the interlocking, is located at the level of the lower⅓ of the height of the profile wire and of the fastener wire, or lower.This interlocking from below makes it possible to avoid the problem ofthe internal sheath creeping. The height and the cross section of thefastener wire contributes to the lack of radial contact between thefastener wire and the profile wire; consequently, the fastener wireabsorbs and transmits the pressure stresses to the outside of the vault.The fastener wire is dimensioned so as to allow it to withstand thestresses due to the internal pressure without the contribution of theprofile wire; thus, there is no substantial transmission of stresses tothe interlocking projections. It is advantageous for the cross sectionof the fastener wire to be similar to the cross section of the profilewire. In practice, the ratio of the cross section of the profile wire tothe cross section of the fastener wire is between 1 and 2 (andpreferably between 1 and 1.6), whereas in the prior art this ratio isgreater than 2 and very often greater than 4 or 5. Likewise, the ratioof the moment of inertial I_(yy) of the profile wire to the moment ofinertia I_(yy) of the fastener wire is advantageously between 1 and 2,thereby allowing the mean moment of inertia of the pressure vault not tobe too greatly reduced compared with that of the profile wire alone.Advantageously, the height of the fastener wire is the same orapproximately the same as the height of the profile wire. This makes itpossible to have a constant outside diameter and a better distributionof the stresses.

Advantageously, for a pipe used as a riser, a thin hoop reinforcementlayer is wound on top of the pressure vault, at the top of the riser.

The invention is applicable to several kinds of wire cross sections.However, it is advantageous for the profile wire of the pressure vaultto have a slimmed-down, I-shaped cross section, and in particular across section in the derived form of a “psi” ψ. The fastener wireadvantageously has a T-shaped cross section.

The invention will be more clearly understood with the aid of thedescription which follows, with reference to the appended schematicdrawings showing, as examples, embodiments of the flexible pipeaccording to the invention. Further advantages and features will becomeapparent on reading the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing the successive layers ofa pipe (in this case of the smooth-bore type) to which the inventionapplies.

FIG. 2 is a partial view in longitudinal section of the pressure vaultof a flexible pipe according to the invention, with a “psi”-shapedprofile wire.

FIGS. 3 and 4 illustrate two alternative embodiments of pressure vaultsaccording to the invention.

FIGS. 5 to 8 illustrate pressure vaults known from the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS

As FIG. 1 shows, and in general, a pipe of the smooth-bore typecomprises, from the inside outwards, a polymeric internal sealing sheath1, a metal vault 2 consisting of the winding of at least one interlockedmetal profile wire in a helix, an armour layer resistant to the axialtension in the longitudinal direction of the pipe and usually consistingof one or more pairs of crossed plies 3, 4 wound in opposite directions,and a polymeric external sealing sheath 5. Other layers (not shown) maybe provided, depending on the type and the application of the pipe, suchas, for example, an internal carcass underneath the internal sealingsheath 1 (for rough-bore pipes), a hoop reinforcement layer consistingof a winding with a short pitch of a rectangular wire, interposedbetween the pressure vault 2 and the first armour ply 3, andintermediate sheaths placed between various armour plies.

FIGS. 5 to 8 show known pressure vaults from the prior art. FIG. 5 showsan unsymmetrical “zeta” profile wire with self-interlocking from above,FIG. 6 shows a “teta” profile wire interlocked by a U from above, FIG. 7shows a self-interlocking T-shaped profile wire and FIG. 8 shows aprofile wire whose interlocking regions are unsymmetrical. The problemwith all these pressure vaults is creep into the gaps. It is thisproblem that the invention shown in FIGS. 2 to 4 solves.

As shown in FIG. 2, the pressure vault 2 consists of a winding of a“psi”-shaped profile wire 10 interlocked by a fastener wire 20. Thefastener wire 20 has in this case approximately the same height as theprofile wire 10 and both are shaped so as to avoid any radial contactbetween the fastener wire and the profile wire in the normal situation,so as to avoid cracking at the interlocking projections.

The profile wire 10 has a profile in the form of an upside-down “psi”,consisting of a web 12, relatively wide external flanges 13 (that is tosay flanges turned towards the outside of the pipe) forming a straightbase 14 (a straight base in cross section, therefore corresponding to acylindrical curved surface) and internal flanges 15 separated by acentral block 16. The internal flanges 15 terminate in an interlockingrim or projection 17 facing the inside of the pipe and separated fromthe central block 16 by a straight flange bottom region 18.

The fastener wire 20 has a profile also consisting of a web 22,relatively short external flanges 23 forming a straight base 24 andinternal flanges 25 forming a straight base 26. The internal flanges 25terminate in an interlocking rim or projection 27 facing the outside ofthe pipe and separated by a straight flange bottom region 28 from aninclined portion 29 for joining to the web 22.

The widths of the flange bottoms 18, 28 are approximately equal andallow positional movement between the profile wire 10 and the fastenerwire 20, between a position of attachment (between the left-hand part ofthe profile wire 10 in FIG. 2 and the fastener 20) and a position ofseparation (between the right-hand part of the profile wire 10 in FIG. 2and the fastener 20).

The height of the central block 16 with respect to the flange bottom 18defines the upper limit of the interlocking region and is less than orequal to one third of the height of the profile wire (for example inthis case 6.4 mm for a profile wire height of 21 mm).

The difference in height between the central block 16 and theinterlocking projection 27 leaves a clearance “e”, avoiding contact andwear by rubbing. If, as a result of the dimensional variations owing tothe tolerances, the deformations and the laying operation, etc.,contacts do initially appear, they disappear with wear. Even in the caseof contacts, the fastener wire practically does not work in bending,because of its design, and the alternating stresses are generated onlyby the rubbing, that is to say by very point-like applications ofpressure. The stresses are therefore always of the same level (withconstant friction coefficient and pressure) whatever the level ofdynamic stressing, which is completely different from the solutions ofthe prior art. Consequently, the fatigue behaviour of the fastener willbe considerably less sensitive to the initial position of the profilewire (contact) and to the variations in dynamic stressing.

The width of the outer base 14 of the profile wire (for example 28.6 mm)is approximately equal to the width of the inner base 26 of the fastenerwire (for example 29 mm) and the width of the inner block 16 (forexample 14 mm) is approximately equal to the width of the outer base 24of the fastener (for example 14.2 mm) so as to balance the internal andexternal pressures.

In the embodiment shown, the moment of inertia I_(xx) and the moment ofinertia I_(yy) of the profile wire are 14536 mm⁴ and 15165 mm⁴respectively and those of the fastener wire are 13035 mm⁴ and 11319 mm⁴respectively. The respective areas of a cross section of the profilewire and of the fastener wire are 356.3 mm² and 289.3 mm². It may beseen in this example that the ratio of the moments of inertia I_(yy) orthe ratio of the areas of a cross section of the profile wire withrespect to those of the fastener wire are close to 1.

FIGS. 3 and 4 show two alternative embodiments of a pressure vaultaccording to the invention.

According to FIG. 3, the profile wire 10 has a “teta”-shaped profileinteracting with a T-shaped fastener wire (with no recesses)interlocked, from below, under a third of the height of the vault. Thearea and the moment of inertia I_(yy) of the profile wire 10 are in thiscase substantially greater than those of the fastener wire 20, howeverthey are in a ratio of less than 2.

According to FIG. 4, the profile wire 10 has been “slimmed down” inorder to give it a “psi” shape so that it moment of inertia is similarto that of the fastener wire 20, the latter being identical wire in FIG.3.

At the top of a riser, the internal pressure is very much greater thanthe external pressure. The pressure vault must absorb large stresseswhich, combined with the dynamic stressing, may cause fatigue problemsin this region of the riser. To alleviate this problem, a first solutionis to give some cross section to the profile wire and to the fastenerwire, as proposed in FIGS. 3 and 4. Another solution is to wind over thetop of the pressure vault a hoop reinforcement 30 of very smallthickness compared with the vault, as shown by dashed lines in FIG. 2.The thickness is around 5 mm. This hoop reinforcement layer 30 may beintroduced only at the top of the riser (where it is needed) and may bereplaced elsewhere with a plastic filling or not be replaced at all.Another advantage of this solution is that the hoop reinforcement can beused to limit the creep of any anti-collapse sheath on top of thepressure vault.

1. Flexible tubular pipe comprising from the inside outwards at least,an internal sealing sheath of a polymer material, a cylindrical pressurevault having an external face and having an internal face placed overthe internal sheath, the vault comprising a winding in a helix with ashort pitch and with a gap between successive turns of the winding, ametal profile wire with successive turns interlocked from below by afastener wire, at least one ply of tensile armour layers wound with along pitch, and an external protective sealing sheath made of a polymer,the fastener wire has substantially the same height as the profile wire,the profile wire and the fastener wire being interlocked, with nonominal radial contact, at most to one third of the height of theprofile wire; and the ratio of a cross section of the profile wire to across section of the fastener wire is between 1 and
 2. 2. Pipe accordingto claim 1, wherein the ratio of the cross section of the profile wireto the cross section of the fastener wire is less than 1.6.
 3. Pipeaccording to claim 1, wherein the wires have respective moments ofinertia and the ratio of the moment of inertia I_(yy) of the profilewire to the moment of inertia I_(yy) of the fastener wire is between 1and
 2. 4. Pipe according to claim 1, further comprising a thin hoopreinforcement wound on top of the pressure vault.
 5. Pipe according toclaim 1, wherein the cross section of the profile wire is “psi” shaped.6. Pipe according to claim 2, wherein the wires have respective momentsof inertia and the ratio of the moment of inertia I_(yy) of the profilewire to the moment of inertia I_(yy) of the fastener wire is between 1and
 2. 7. Pipe according to claim 2, further comprising a thin hoopreinforcement wound on top of the pressure vault.
 8. Pipe according toclaim 3, further comprising a thin hoop reinforcement wound on top ofthe pressure vault.
 9. Pipe according to claim 6, further comprising athin hoop reinforcement wound on top of the pressure vault.
 10. Pipeaccording to claim 2, wherein the cross section of the profile wire is“psi” shaped.
 11. Pipe according to claim 3, wherein the cross sectionof the profile wire is “psi” shaped.
 12. Pipe according to claim 4,wherein the cross section of the profile wire is “psi” shaped.
 13. Pipeaccording to claim 6, wherein the cross section of the profile wire s“psi” shaped.